It has long been known to employ organic polymer materials for obtaining ophthalmic lenses.
One of the materials most commonly employed is obtained by polymerization of the monomer CR 39200 (diethylene glycol diallyl carbonate).
However, for a number of years, there has been an increasing demand for lenses which, while guaranteeing substantially the same optical qualities and the same mechanical characteristics as the lenses made of known organic polymer material, are thinner and are therefore lighter in weight.
Various materials have been developed to meet this demand. These new organic materials generally have high refractive indices which can reach, or even exceed, refractive indices n=1.6.
Thus, as a result of this high refractive index, it is possible to manufacture lenses of lower thickness for an equivalent corrective power (optical power).
A class of monomers that have recently been tested for obtaining materials in ophthalmics is the class of thio(meth)acrylates.
Patent EP-A-273 661 describes monomers of thio(meth)acrylate type which can be used in the field of ophthalmic optics, and patent EP-A-273 710 describes polymerizable compositions producing transparent organic polymers which are suitable for the production of ophthalmic lenses. These compositions contain a monomer which has at least one thio(meth)acryloyl group and at least two (meth)acryloyl groups and, optionally, at least one other monomer which is copolymerizable by a radical route.
Patent EP-A-273 710 describes in particular compositions based on bis(2-methacryloylthioethyl) sulfide.
This patent also mentions that the polymerizable compositions can be polymerized by a thermal route, by UV irradiation or by the action of an electron beam.
All the compositions given by way of example in this patent are polymerized either by a purely thermal route (for several hours), or by prepolymerization using UV irradiation followed by a thermal polymerization to obtain a hardened moulded article.
Patent application WO-90/04587 describes 4,4'-bis [(meth)acryloylthiophenyl] sulfide monomers and their use in poymerizable compositions in combination with a copolymerizable vinyl monomer and/or polythiols.